Nonlinear and time-resolved optical study of the 112-type iron-based superconductor parentCa1−xLaxFeAs2across its structural phase transition

Abstract

The newly discovered 112-type ferropnictide superconductors contain chains of As atoms that break the tetragonal symmetry between the a and b axes. This feature eliminates the need for uniaxial strain that is usually required to stabilize large single domains in the electronic nematic state that exists in the vicinity of magnetic order in the iron-based superconductors. We report detailed structural symmetry measurements of 112-type Ca 0.73 La 0.27 FeAs 2 using rotational anisotropy optical second-harmonic generation. This technique is complementary to diffraction experiments and enables a precise determination of the point-group symmetry of a crystal. By combining our measurements with density functional theory calculations, we uncover a strong optical second-harmonic response of bulk electric dipole origin from the Fe and Ca 3d-derived states that enables us to assign C 2 as the crystallographic point group. This makes the 112-type materials high-temperature superconductors without a center of inversion, allowing for the possible mixing of singlet and triplet Cooper pairs in the superconducting state. We also perform pump-probe transient reflectivity experiments that reveal a 4.6-THz phonon mode associated with the out-of-plane motion of As atoms in the FeAs layers. We do not observe any suppression of the optical second-harmonic response or shift in the phonon frequency upon cooling through the reported monoclinic-to-triclinic transition at 58 K. This allows us to identify C 1 as the low-temperature crystallographic point group but suggests that structural changes induced by long-range magnetic order are subtle and do not significantly affect electronic states near the Fermi level.

Phase stability and large in-plane resistivity anisotropy in the 112-type iron-based superconductor Ca1−xLaxFeAs2

“…the space group P 2 1 or P 2 1 /m rather than the originally assumed tetragonal structure [13,14]. Second harmonic generation experiments confirmed the space group P 2 1 for La doped compounds [15]. More recently, theoretical studies focusing on the spacer layers determined that the As p x and p y orbitals are responsible for the Dirac cones, and the spin orbit coupling can open a gap and induce topological phases on these layers, suggesting the 112 compounds as prime candidates for proximity induced topological superconductivity [16,17].…”

“…(The measured lattice constants are a = 3.9163Å, b = 3.8953Å, c = 10.311Å, and β = 90.788 • .) The strong optical second-harmonic response was recently observed in Ca 1−x La x FeAs 2 , clearly implying that the crystal does not have the inversion symmetry [15], but similar data does not exist for Ca 1−x Pr x FeAs 2 to the best of our knowledge. Hence, the first principles predicted structure is quite similar to the experimental structure, except that it does not capture the possible inversion symmetry breaking.…”

“…[13]. While second harmonic generation verifies the lack of inversion symmetry in the La doped compound [15], no similar study exists for the Pr doped compound.) There is a group-subgroup relationship between these two space groups, and the only difference between the structures is a polar distortion, discussed at the end of this subsection.…”

A parity-breaking electronic nematic phase transition in the spin-orbit coupled metal Cd 2 Re 2 O 7

“…Because of this property, optical SHG has been used recently to study noncentrosymmetric Weyl semimetals (31) and odd-parity order in correlated iridates and cuprates (32,33). To completely resolve the structure of for Cd2Re2O7, we used a recently-developed high-speed rotational anisotropy (RA) technique (34,35) that involves focusing a beam of light obliquely onto the surface of a single crystal and measuring variations in the intensity of reflected SHG light as the scattering plane is rotated about the surface normal. By projecting the SHG signal radiated at different scattering plane angles onto a circular locus of points on a stationary 2D detector ( Fig.…”

Arsenic chemistry of iron-based superconductors and strategy for novel superconducting materials

“…According to single crystal X-ray structure analysis, Sb substitutes preferably to As in zigzag chains, rather than As in the FeAs layer [45], as predicted by first-principles calculations [46]. In contrast, excess doping of La causes antiferromagnetic and structural phase transitions [47][48][49], which results in loss of superconductivity. The observation of optical second-harmonic generation confirmed the lack of spatial inversion symmetry for Ca 1−x La x FeAs 2 [49], which makes the 112-type a unique among iron-based superconductors.…”